OOGENESIS 31 



oocyte. The other cell, which is relatively very small, is known as the 

 first polar body, or polar cell. It has the same nuclear contents as the sec- 

 ondary oocyte, and may divide into two other polar bodies, equivalent to 

 mature ova. More often it degenerates without division. When the 

 secondary oocyte divides, it likewise produces one large cell, the mature 

 ovum, and one small cell, the second polar body. The latter is said to be cap- 

 able of fertilization, but to what extent it may develop is unknown. 

 Functionally the production of polar bodies serves to prevent the sub- 

 division and distribution of the nutritive material elaborated within the 

 primary oocyte. One mature ovum with abundant yolk is provided at 

 the expense of three ova (polar bodies) which degenerate. 



Although the maturation of the ovum has not been observed in man, 

 nor even the presence of definite polar bodies, the entire process has been 



Oogonia 



Polar Bodies 



Secondary Oocyte 



Mature Ovum 



FIG. 23. DIAGRAM OF THE CELL DIVISIONS IN OOGENESIS. (Compare with Fig. 18.) 



carefully studied in other mammals, notably in the mouse. 1 It has been 

 shown that the maturation of the ovum of the mouse takes place rapidly, 

 both of the oocyte divisions being accomplished within from four to fifteen 

 hours. The first polar body usually forms before the oocyte is discharged 

 from the ovarian follicle in other words, before ovulation takes place. 

 The second polar body is usually formed in the uterine tube, after the sper- 

 matozoon has entered the oocyte. Long and Mark have found that the 

 chromosomes of the primary oocyte are tetrads, or bodies showing trans- 

 verse and longitudinal divisions; and that those of the secondary oocyte are 

 dyads. They believe that the first division is transverse or reductional, 

 and that the second is equational. 



1 Among the most important papers are: Sobotta, J., Die Befruchtung und Furchung des 

 Eies der Maus. Arch. mikr. Anat., 1895, vol. 45, pp. 15-91. 



Long, J. A., and Mark, E. L. The maturation of the egg of the mouse. Carnegie Inst. 

 Publ. No. 142, 1911, pp. 1-72. 



